teaching the concept of density, a play in two acts:

Just before Columbus Day, we tried to teach the kids at the nursery school about Christopher Columbus and his three ships. The emphasis was on Columbus' bravery, his importance to the history of the United States, and, since we live in a predominantly Italian neighborhood, his ethnic background.

To illustrate the mechanics of "sailing," we had the kids build little boats by sticking a coffee stirrer into a bar of Ivory soap and affixing a construction paper "sail." We created a miniature "ocean" using giant plastic tubs that we filled with water. The kids put the boats in the water and blew on the sails to send them across the ocean.

Of course, giant plastic tubs, when placed in the context of a three-year-old mind, really do look like oceans, and their hands, arms, and faces represent giant monster appendages just ripe for the dunking. So we pushed up sleeves, stepped back, and let them have at it.

While they were splashing around, we provided various examples of "obstructions" that Columbus may have encountered during his sail, such as rocks, ice (bergs) cubes, ... wadded up paper towels... orange candles... crayons... pretty much anything that wasn't nailed down. This gave us a great opportunity to discuss with the children the concept of density, or relative water displacement. Do you think the rocks will sink? was followed by a resounding chorus of YES and a couple of timid NOs.

During the morning session, I was lucky enough to be teaching in the adjoining room, and was privy to dialogue led by my (genius, seriously) cousin who was "in charge" of the lesson:

Teacher: So, which do you think is more dense? The rock? Or the candle?
Children: THE CANDLE
Teacher: Really?
Children: THE ROCK
Teacher: That's right! That's because the ratio of the mass to the volume of the rock is greater than that ratio in the candle. Understand?
Children: HEY, THIS ROCK FLOATS! HA HA! LOOK AT THIS FLOATING ROCK!
Teacher: That's because that rock has air bubbles inside, effectively removing some of the mass. It is less dense than the other rocks.

During the afternoon session, however, I was switched to the (now completely waterlogged) "ocean room" to lead the lesson for the second group of kids.

Kate: So, which do you think will float? The rock? Or the candle?
Children: THE CANDLE
Kate: Really?
Children: THE ROCK
Kate: That's right! That's because I got this special floating rock from the moon. Most rocks will sink, but I brought back this special floating rock just for you guys.
Children: YOU'VE NEVER BEEN TO THE MOON!
Kate: Then explain to me how this rock floats, smarty-pants! (with that, I dropped the rock in the water and watched as their faces lit up.)
Children: WOW! I WANT TO PLAY WITH THE MOON ROCK! GIVE ME THE MOON ROCK! CAN I SEE THE MOON ROCK NOW?

Which teaching method is better? You be the judge. Personally, I think a three-year-old is a lot more likely to remember that most rocks usually sink in the context of their teacher's moon rock not sinking, but maybe I'm wrong and a more complex definition of relative density was the way to go.